1. Field of the Invention
This invention relates to a low impurity hexagonal würtzite type epitaxial layer, and more specifically, a low alkali-metal epitaxial layer, grown on a hexagonal würtzite substrate, and a method for creating the low impurity content hexagonal würtzite type epitaxial layer.
2. Description of the Related Art
The usefulness of gallium nitride (GaN), and its ternary and quaternary compounds incorporating aluminum and indium (AlGaN, InGaN, AlInGaN), has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially using growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).
GaN and its alloys are the most stable in the hexagonal würtzite crystal structure, in which the structure is described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axis), all of which are perpendicular to a unique c-axis. Group III and nitrogen atoms occupy alternating c-planes along the crystal's c-axis. The symmetry elements included in the würtzite structure dictate that III-nitrides possess a bulk spontaneous polarization along this c-axis, and the würtzite structure exhibits piezoelectric polarization.
Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, conventional c-plane quantum well structures in III-nitride based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. The strong built-in electric fields along the c-direction cause spatial separation of electrons and holes that in turn give rise to reduced carrier recombination efficiency, reduced oscillator strength, and red-shifted emission.
One approach for eliminating or reducing the spontaneous and piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on non-polar or semi-polar planes of the crystal. Recently, several reports have been published which confirmed the benefit of the non-polar and semi-polar devices. Most of them indicate that a high-quality substrate is essential for fabrication of these devices. Historically, numerous foreign substrates, such as SiC, spinel, sapphire, etc., have been used to fabricate devices, however, the device quality has been poor due to the high defect density caused by heteroepitaxial growth of the devices on the foreign substrates.
Accordingly, high quality and high cost-performance GaN substrates are necessary for homo-epitaxial growth and industrialization of non-polar and semi-polar devices. One approach is to utilize HVPE GaN substrates for the production of non-polar and semi-polar devices, but the substrate wafer size is limited, and also the production cost is quite high.
Currently, there are a variety of methods being pursued for the growth of GaN single crystal substrates. In many approaches, one or more alkali-metals are needed and intentionally added into the growth system, for example, sodium and/or potassium are added for the growth of GaN in supercritical ammonia, and sodium is a key constituent in the growth of GaN by the sodium flux method. This addition of alkali-metal to the growth environment causes the grown GaN crystals to typically contain high concentrations of the alkali-metals that are used during the growth process, and this severely increases the likelihood that the alkali-metals in the substrate will diffuse into the epitaxial layers during device growth and have a negative impact on the device electrical properties and performance.
The present invention provides a hexagonal würtzite type epitaxial layer possessing a low concentration of alkali-metals. The present invention also provides a method to produce these low impurity content epitaxial films by minimizing the diffusion of the alkali-metals from the substrate into the epitaxial film by choosing a proper crystal plane of the substrate.
The term “semi-polar planes” can be used to refer to a wide variety of planes that possess two nonzero h, i, or k Miller indices; and a nonzero l Miller index. The term “non-polar planes” can be used to refer to a wide variety of planes that possess two nonzero h, i, or k Miller indices; and a zero l Miller index.